The possibility to determine the position of a mobile device has enabled application developers and wireless network operators to provide location based, and location aware, services. Examples of those are guiding systems, shopping assistance, friend finder and other information services giving the mobile user information about their surroundings.
In addition to the commercial services, the governments in several countries have also put requirements on the network operators to be able to determine the position of an emergency call. For instance, the governmental requirements in the USA (FCC E911) require that it must be possible to determine the position of a certain percentage of all emergency calls. There is no difference between the requirements put on indoor environments compared to outdoor environments.
In outdoor environments, the position estimation can be done using external methods for position determination, e.g. GPS (Global Positioning System) based methods like Assisted-GPS (A-GPS). Position estimation can also be performed using the wireless network itself. Methods using the wireless network can be grouped in two main groups. The first group comprises methods that are based on the radio cell to which a mobile terminal is attached, e.g. by using Cell-ID or E-CGI (Enhanced Cell Global Identity). The second group uses measuring of radio signals from several base stations (BS) and determining the terminal position using e.g. Time Difference (TD).
In order to be able to connect to a mobile network or to perform handover when connected, a mobile terminal typically constantly measures available signals, not only from its own base station, but also from other base stations. These signals are typically control signals intended for measuring radio conditions of transmissions, which control signals contain, among other data, information about how to establish a connection to the transmitting base station. In particular, the control signals comprise data, which by itself or in combination with the frequency of the carrier on which the control signal was transmitted constitute base station identification data. A mobile terminal can thus obtain an identity of the transmitting base station and an estimate of the radio conditions. The mobile terminal typically compiles this information, in GSM (Global System for Mobile communications) in a neighbor list, which is transferred to the network as information.
Position estimation can be based on measurements in the neighbor list. One then uses the relation between the distance from the radio base station and the radio condition in combination with knowledge about the exact position of the base station. The base station positions are known within the communications network. This means that the neighbor list easily can be used for position estimating according to different algorithms. The accuracy of the position estimation is generally proportional to the size of the cell.
Triangulations, or Time Difference (TD) methods, use signals associated with two or more different base stations. These signals are used to calculate the position or at what distance from the base station a mobile terminal is located. The calculations are based on the relative or absolute difference in the time it takes the signal to propagate between the terminal and the different base stations. The achievable accuracy of TD-methods depends on system architecture, physical conditions and radio conditions. Typically, the accuracy of a TD method a mobile telephony system is 50 to 150 meters. TD methods are also relatively time and resource consuming.
Fingerprinting methods use the fact that all places have a, more or less, unique characteristic signature of the received radio signals. This is the result of multi-pathing and reflections in the buildings and obstacles. By storing the characteristic radio signature of different locations in a database, it is possible determine the location of a device by comparing the received signature of a signal with the signatures stored in the database. Fingerprinting methods require an always-updated database. A good result typically also relies on being able to match signals from several different sources or base stations.
A terminal located indoors typically has a connection to a base station covering the surrounding outdoor area that is of lower quality than if the terminal would have been located outdoors. To improve the indoor coverage situation, many larger buildings are equipped with an indoor mobile telephony system. The indoor system most often consists of one base station and a distributed antenna system or a leaking cable antenna. For buildings spread over large areas repeaters are typically used. This results in the entire building appearing as one large radio cell making it impossible to determine where the terminal is located within the building. Furthermore, due to weak signals from base stations located outdoors, more sophisticated methods using, e.g., triangulation, are normally not practical to apply.
One solution is to use an additional system for positioning, a system that is not based on any mobile telephony system. This can be an indoor GPS system, a WLAN (Wireless Local Area Network) or a Bluetooth based system or some other sensor solution. However, such systems require additional complex equipment, and also the terminals have to be equipped with special hardware and/or software, which makes the solution expensive.
Another solution is to increase the number of indoor base stations, thus reducing the size of the cells. Such a solution will also increase the total available communication resources, since there typically are possibilities for more efficient reuse of communication resources. However, a base station is an expensive piece of equipment and such a solution will therefore be very costly. If the desire of an improved position determination is the only reason for increasing the number of base stations, the investments are typically unreasonably high.
Furthermore, when dividing a large cell into smaller cells, the number of available carrier frequencies in each small cell is typically a fraction of the available carrier frequencies in the large cell corresponding to the reuse factor. For situations, where the traffic intensity varies significantly with time and/or between different small cells, the dynamics of the resource allocation is detrimentally reduced.